Ultrathin, conformal polymer coatings as separators at nanostructured metal oxides used for energy storage

ABSTRACT

An electrode for use in energy storage comprising a nanostructured mesoporous electrically conductive metal oxide and an ultrathin, conformal polymer coating on the metal oxide wherein said electrode has a mesoporous structure. Also disclosed is the related method for making an electrode for use in energy storage.

BACKGROUND

1. Field of the Invention

The present invention relates to energy storage, such as inultracapacitors and solid-state batteries, and, more specifically, toenergy storage with power sources that use acidic electrolytes.

2. Description of the Prior Art

Aerogels (and related ambigels) are sol-gel-derived nanoarchitecturescomposed of a three-dimensional network of nanoscale particlesintermingled with a continuous, aperiodic mesoporosity. Thearchitectural characteristics of high surface area and continuousporosity enhance the transport of ions and molecules throughout thepore-solid architecture for interaction with the nanoscopic soliddomains. This combination of properties, which are intrinsic toelectrically conductive aerogels, makes them attractive candidates aselectrode materials for energy-storage devices including batteries andultracapacitors. Aerogels and ambigels based on metal oxides, such asmanganese oxide (MnO₂), are particularly relevant for charge storage, assuch oxides undergo reversible cation-electron insertion reactions.Nanostructured metal oxide electrodes exhibit superior performance whenused as lithium-battery electrodes or ultracapacitors. See D. R. Rolison& B. Dunn, J. Mater. Chem., 11, 963 (2002), incorporated herein byreference.

Ultracapacitors are a class of energy-storage materials that offersignificant promise in bridging the performance gap between the highenergy density of batteries and the high power density derived fromdielectric capacitors. Currently, high-performance ultracapacitors arebased on nanoscale forms of mixed ion-electron conducting metal oxides,such as RuO₂, which store charge via a cation-electron insertionmechanism.Ru^(IV)O₂+xe⁻⁺ xH ⁺

H_(x)Ru^(III) _(x)Ru^(IV) _(1-x)O₂  [Equation 1]The charge/discharge profiles associated with such reactions often mimicthose of capacitors with a constant charge released or stored over abroad potential range, and thus this type of charge storage is oftendesignated as pseudocapacitance. Ultracapacitors based on hydrous RuO₂yield specific capacitances as high as 768 F/g. The application of RuO₂is limited however by its high cost as a platinum-group metal and itsnon-domestic sources.

The abundance of manganese minerals and the low toxicity of manganeseprecursors make MnO₂ both an economical and an environmentally benignalternative to RuO₂. Manganese oxides are well-studied materials for useas insertion electrodes, with applications ranging from alkaline Zn/MnO₂cells (equation 2) to lithium-ion batteries (equation 3).Mn^(IV)O₂ +xe ⁻ +xH ⁺

H_(x)Mn^(III) _(x)Mn^(IV) _(1-x)O₂  [Equation 2]Mn^(IV)O₂+xe⁻+xLi⁺

Li_(x)Mn^(III) _(x)Mn^(IV) _(1-x)O₂  [Equation 3]Manganese oxides may also be synthesized in a wide range of polymorphs,each with characteristic electrochemical properties. Manganese oxideshave been investigated as ultracapacitor electrodes in neutral aqueouselectrolytes. Specific capacitance values for MnO₂ are as high as 700F/g for thin-film electrodes, although practical MnO₂ electrodeconfigurations yield only 200 F/g. See S. C. Pang, M. A. Anderson & T.W. Chapman, J. Electrochem. Soc., 147, 444 (2000); H. Y. Lee & J. B.Goodenough, J. Solid State Chem., 144, 220 (1999); and J. W. Long, A. L.Young & D. R. Rolison, Advanced Batteries and Super Capacitors, G.Nazri, R. Koetz, B. Scrosati, P. A. Moro, E. S. Takeuchi (Eds.) PV2001-21, Electrochemical Society (Pennington, N.J.), 2003, pp. 752–759,all of which are incorporated herein by reference.

Previous studies with hydrous RuO₂ have demonstrated that the maximumultracapacitance is achieved in acidic electrolytes, where highconcentrations of highly mobile protons are available to the oxideelectrode. See L. D. Burke, O. J. Murphy, J. F. O'Neill & S. Venkatesan,J. Chem. Soc., Faraday Trans., 73, 1659 (1977) and E. W. Tsai & K.Rajeshwar, Electrochim. Acta, 36, 27 (1991), both of which areincorporated herein by reference. However, manganese oxide undergoes areductive-dissolution process when exposed to even mildly acidicelectrolytes, yielding water-soluble Mn(II) species.Mn^(IV)O₂+H⁺+e⁻

Mn^(III)OOH  [Equation 4]2Mn^(III)OOH+2H⁺→Mn^(IV)O₂+Mn^(II)(soluble)+2H₂O  [Equation 5]Redeposition of MnO₂ via electro-oxidation of Mn(II) is inhibited inacid electrolytes, requiring high overpotentials and elevatedtemperatures to achieve significant deposition rates. The use of MnO₂ asan ultracapacitor, therefore, is limited to near-neutral-pH aqueouselectrolytes where the pseudocapacitance is restricted by the presenceof less-desirable insertion cations, such as Li⁺ and K⁺, which competewith H⁺ for association at the MnO₂ electrode.

Conducting polymers are also being investigated as ultracapacitorsbecause of their ability to undergo electrochemically drivenion-insertion reactions. See A. Rudge, J. Davey, I. Raistrick, S.Gottesfeld & J. P Ferraris, J. Power Sources, 47, 89 (1994),incorporated herein by reference. The energy density of ultracapacitorsbased on conducting polymers is restricted by the low mass-density ofthe active organic component as well as the low ion-doping levels,typically less than 0.5 electrons/ions per monomer unit. This limitationof conducting polymer ultracapacitors can be somewhat offset by pairingp-doped and n-doped polymer electrodes in nonaqueous electrolytes, wherehigher cell voltages (2–3 volts) can be achieved. However, theseelectrolytes have the further disadvantages of cost and flammabilityrelative to aqueous acid electrolytes.

SUMMARY

We extend the versatility of electrically conductive aerogels andrelated structures by modifying them with insulating and conductingpolymers. By choosing self-limiting electropolymerization schemes thatresult in conformal, ultrathin (<15-nm thick) polymer coatings wegenerate hybrid structures that retain the desirable properties ofsurface area and continuous mesoporosity inherent to the initialmetal-oxide or carbon nanoarchitecture. Representative electrodepositedpolymers include poly(o-phenylenediamine), PPD, andpoly(o-methoxyaniline), POMA. In a preferred embodiment, the metal oxideis manganese or iron oxide. Another aspect of the present invention is amethod of making a high-performance aqueous-acid ultracapacitorcomprising the steps of (a) preparing a nanostructured, mesoporous metaloxide film, and (b) electrochemically depositing an ultrathin polymercoating on the metal oxide film.

The present invention has the potential to deliver high volumetriccharge-storage density, particularly when compared to ultracapacitorsbased exclusively on conducting polymers, with economic advantages dueto the much lower costs of manganese and iron oxides relative to currenthigh-performance ultracapacitors based on ruthenium oxides.

The present invention serves as the platform for an all-solid-state,three-dimensionally constructed battery where the metal oxide functionsas the battery cathode and the insulating polymer coating functions asthe separator/electrolyte. Battery fabrication will be completed byfilling the remaining pore volume of the polymer-metal-oxide hybridstructure with an appropriate anode material, such as metallic lithium.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the invention, aswell as the invention itself, will become better understood by referenceto the following detailed description, appended claims, and accompanyingdrawings where:

FIG. 1 shows scanning electron micrographs for (a) native, uncoated MnO₂ambigel film electrode, and (b) PPD-coated MnO₂ ambigel film electrode;

FIG. 2 shows Conducting-mode atomic force microscopy images for (a) anative, uncoated MnO₂ ambigel film electrode, and (b) a PPD-coated MnO₂ambigel film electrode. For the native, uncoated MnO₂ film the measuredconductivity (tunneling current) coincides with the MnO₂ solid domainsof the porous electrode. In the case of the PPD-coated MnO₂ electrode nomeasurable tunneling currents are found and no imaging is possible. Thisinability to image is indicative of the highly insulating nature of thePPD polymer coating, and the complete coverage of the high-surface-areaMnO₂ electrode by this polymer;

FIG. 3 shows cyclic voltammograms for (a) native MnO₂ ambigel film, and(b) PPD-coated MnO₂ ambigel film in 0.1 M H₂SO₄ and the absorbance ofthe MnO₂ film at 500 nm simultaneously recorded during the voltammetricscan. In the case of the native MnO₂ film, electrochemical reduction isirreversible as indicated by the absence of current peaks on the reversevoltammetric scan. The absorbance at 500 nm also tracks the totaldissolution of the MnO₂ film on the first reduction sweep. For thePPD-coated MnO₂ film, no electrochemical dissolution occurs at theexpected potentials. The reduction current at E<0 V and the accompanyingloss in absorbance is attributed to proton insertion at the underlyingMnO₂, controlled by the redox reactions of the PPD polymer coating; and

FIG. 4 shows a cyclic voltammogram for a POMA-coated MnO₂ ambigel filmin 0.1 M H₂SO₄ and the differential absorbance (dA₅₀₀/dt) of thePOMA-coated MnO₂ film at 500 nm simultaneously recorded during thevoltammetric scan. The differential absorbance provides a direct measureof the MnO₂ electronic state during the voltammetric measurement. Thisresult demonstrates the underlying MnO₂ can be reversibly reduced andre-oxidized beneath the polymer coating, which is contacting an acidelectrolyte.

DETAILED DESCRIPTION

Nanostructured conducting-polymer—metal-oxide hybrids are designed ashigh-energy-density ultracapacitors as well as platforms forall-solid-state, three-dimensionally designed batteries. In the firstcase, the polymer components of these hybrids stabilize normallyunstable oxides, such as MnO₂, Fe₂O₃, and FeOOH, against corrosion inaqueous-acid electrolytes. The polymer also serves as an active protonconductor, supplying charge-compensating protons from the externalelectrolyte to the encapsulated metal oxide domains. The highmass-density and faradaic capacity of the metal oxide provide highenergy density in the resulting hybrids. The success of these hybrids asultracapacitors relies on the ability to reversibly access the availableoxidation states of the oxide component: Mn(IV)

Mn(III)

Mn(II) for MnO₂; and Fe(III)

Fe(II)

for Fe₂O₃ or FeOOH. Polymer-coated, nanostructured oxides can be used asthe active components in low-cost, high-performance aqueous-acidultracapacitors.

Metal oxides that can be used in the present invention include, but arenot limited to, manganese, iron, vanadium, and nickel oxides, ormixtures of these oxides (either physical or intimate chemicalmixtures). In a preferred embodiment, manganese oxides are used.Nanostructured highly porous architectures of manganese oxides can besynthesized as aerogels, ambigels, and xerogels. See J. W. Long, K. E.Swider-Lyons, R. M. Stroud & D. R. Rolison, Electrochem. Solid-StateLett., 3, 453 (2000); and J. W. Long, R. M. Stroud & D. R. Rolison, J.Non-Cryst. Solids, 285, 288 (2001), all of which are incorporated hereinby reference. Nanostructured, mesoporous MnO₂ films are prepared asdescribed in J. W. Long, L. R. Qadir, R. M. Stroud & D. R. Rolison, J.Phys. Chem. B, 105, 8712 (2001), incorporated herein by reference.

In a preferred embodiment, the polymer component is based on anarylamine monomer, such as o-phenylenediamine or aniline. The polymerserves as a physical barrier to an external aqueous acidic electrolyte,specifically the H₂O and hydrated protons therein, while providing fortransport of charge-compensating unsolvated protons to the underlyingmetal oxide via an electrochemical gating mechanism. See P. Burgmayerand R. W. Murray, J. Am. Chem. Soc., 104, 6139 (1982), incorporatedherein by reference. A protective, proton-conductive polymer film isprepared on the MnO₂ electrode by the electrochemically initiatedpolymerization of o-phenylenediamine (OPD). Established methods can beused for the oxidative electropolymerization of OPD. See H. S. White, H.D. Abruña & A. J. Bard, J. Electrochem. Soc., 129, 265 (1982) and A. M.Yacynych & H. B. Mark, Jr., J. Electrochem. Soc., 123, 1346 (1976), bothof which are incorporated herein by reference.

As-prepared films of mesoporous MnO₂ are initially subjected to anelectrochemical oxidation in 0.2 M Na₂SO₄ (pH 9 borate buffer)electrolyte to lower the solid-state concentration of Mn(III) centers inthe mesoporous oxide nanoarchitecture. The electrodes are thentransferred to a second electrolyte containing ˜10 mM of the arylaminemonomer in a basic electrolyte of 0.2 M Na₂SO₄ (pH 9 borate buffer).Polymerization of the monomer can be initiated using a number ofelectrochemical techniques including voltammetric, potentiostatic,galvanostatic, potential-pulse, and current-pulse methods. Theelectrochemical oxidation of OPD monomers to formpoly(o-phenylenediamine) (PPD) commences at a potential of approximately+0.44 V vs. the normal hydrogen electrode. Regardless of theelectrochemical method employed, the growth of the PPD coating isself-limited due to the exceptionally poor electronic conductivity andminimal swelling of the developing polymer. The film thickness of PPDfilms at planar electrodes is typically less than 10 nm, and moretypically 7–9 nm. By applying the polymer coating under self-limitingconditions, the mesoporous oxide structure can be exhaustively coatedwithout filling in the mesopore network (see FIG. 1). Polymer-coatedMnO₂ films are further subjected to heating at 150° C. in a vacuum ovenfor 12 hours to improve the stability of the hybrid structure. Numerousother electropolymerizable monomers are candidates to form self-limited,conformal polymeric films on high-surface-area charge-insertion oxidenanoarchitectures.

The polymer-coated MnO₂ mesoporous electrodes are then electrochemicallyanalyzed in 0.1 M H₂SO₄ electrolyte using voltammetry coupled withsimultaneous spectroscopic measurements, which track changes in theelectronic state of both the MnO₂ and conducting polymer as a functionof potential and state-of-charge. Uncoated MnO₂ electrodes exhibit anirreversible dissolution process during the reduction cycle (see FIG. 3a), and are thus not good candidates for charge storage in acidelectrolytes. When mesoporous manganese oxide electrodes that have beenelectrochemically coated with PPD are subjected to cycling in acid, nofeatures characteristic of dissolution are observed (see FIG. 3 b). Thehighly porous, nanoscopic, high-surface-area oxide architecture isremarkably stabilized to acid-induced dissolution by the ultrathinelectrodeposited polymer.

These acid-stable organic-inorganic hybrids exhibit electrochemicalactivity in the potential range of +0.4 V to −0.4 V. The electrochemicaland spectroscopic responses can be ascribed to electronic-state changesof both the polymer coating and the underlying MnO₂. Although thesepolymer coatings are insulating (see FIG. 2 b) under the basicelectrolyte conditions from which they are electrodeposited, whentransferred to acidic electrolytes these polymers are electro-active andbehave like more conventional conducting polymers. The reversibility forproton-insertion in the encapsulated MnO₂ domains is dictated by theredox potential of the polymer coating. Polymers, such as poly(aniline)(PANI) and poly(o-methoxyaniline) (POMA), have redox potentialsoverlapping those of MnO₂. Reversible proton-insertion in acidelectrolytes is observed for MnO₂ electrodes protected with ultrathinPOMA coatings (see FIG. 4).

The above description is that of a preferred embodiment of theinvention. Various modifications and variations are possible in light ofthe above teachings. It is therefore to be understood that, within thescope of the appended claims, the invention may be practiced otherwisethan as specifically described. Any reference to claim elements in thesingular, e.g. using the articles “a,” “an,” “the,” or “said” is notconstrued as limiting the element to the singular.

1. An electrode for use in energy storage comprising: (a) ananostructured electrically conductive metal oxide interpenetrated by acontinuous mesoporous network; and (b) an ultrathin, conformal polymercoating on the metal oxide; wherein said electrode has a mesoporousstructure and said metal oxide retains said mesoporous network.
 2. Theelectrode of claim 1 wherein said metal oxide is selected from the groupconsisting of manganese oxides, vanadium oxides, nickel oxides, ironoxides, and physical or compositional mixtures thereof.
 3. The electrodeof claim 1 wherein said polymer coating is based on an aryl monomer thatcan be electrochemically oxidized under self-limiting conditions.
 4. Theelectrode of claim 1 wherein said polymer coating is selected from thegroup consisting of o-phenylenediamine, aniline, and mixtures thereof.5. The electrode of claim 1 wherein said metal oxide has a porousarchitecture and wherein said polymer coating is depositedelectrochemically in a self-limiting fashion on said metal oxide suchthat said metal oxide architecture retains said porosity.
 6. Theelectrode of claim 1 wherein said polymer coating is less than 10-nmthick.
 7. A method for making an electrode for use in energy storage,comprising the steps of: (a) preparing a nanostructured electricallyconductive metal oxide interpenetrated by a continuous mesoporousnetwork; and (b) depositing a polymer coating on the metal oxide film;wherein said electrode has a mesoporous structure and said metal oxideretains said mesoporous network.
 8. The method of claim 7 wherein saidmetal oxide is selected from the group consisting of manganese oxides,vanadium oxides, nickel oxides, iron oxides, and physical orcompositional mixtures thereof.
 9. The method of claim 7 wherein saidpolymer coating is based on an arylamine monomer.
 10. The method ofclaim 7 wherein said polymer coating is selected from the groupconsisting of o-phenylenediamine, aniline, and mixtures thereof.
 11. Themethod of claim 7 wherein said metal oxide has a porous architecture andwherein said polymer coating is deposited electrochemically in aself-limiting fashion on said metal oxide such that said metal oxidearchitecture retains said porosity.
 12. The method of claim 7 whereinsaid polymer coating is less than 10-nm thick.